Demand for secondary batteries as an energy source is rapidly increasing due to the increase in technological development and demand for mobile devices. Among such secondary batteries, lithium secondary batteries, which have a high energy density and voltage, a long lifetime, and a low self discharge rate, have been commercialized and are widely used. However, lithium secondary batteries have a limitation of repeated charge/discharge cycles dramatically reducing the lifetime of the batteries. In particular, the limitation is more severe at high temperatures. This is due to effects which are generated when moisture inside the batteries or the like causes electrolyte decomposition or active material degradation, as well as increased internal resistance of the battery.
Accordingly, a positive electrode active material for lithium secondary batteries which is currently being actively developed and used is LiCoO2, which has a layered structure. LiCoO2 is the most widely used positive electrode active material, having excellent lifetime properties and an excellent charge/discharge efficiency. However, due to having low structural stability, the application of LiCoO2 in battery capacity-increasing techniques is limited.
Various lithium transition metal oxides have been developed as positive electrode active materials for replacing LiCoO2, such as LiMnO2 and Li2MnO3, which have layered structures, LiMn2O4, which has a spinel crystal structure, LiNiO2, LiFePO4, and Li(Nix1Coy1Mnz1)O2.
Among these, lithium manganese-based oxides such as LiMnO2, Li2MnO3, and LiMn2O4, have the advantages of excellent thermal stability and low cost, but also have the limitations of small capacity, and poor high-temperature properties. Specifically, although initially having a layered crystal structure, after a charge/discharge, LiMnO2 is transformed into a spinel crystal structure, and consequently there are limitations of a decreased lithium ion movement rate and a reduction in capacity. LiMn2O4 has been commercialized to a degree as a low-cost product. However, LiMn2O4 has a low capacity due to having a spinel crystal structure, and poor lifetime properties due to a structural deformation—Jahn-Teller distortion—caused by Mn3+. Li2MnO3 has a high Mn content and thus has the advantages of being extremely low-cost and having an extremely large capacity at high voltage, and exhibits an effect in which the capacity increases after a plateau region from 4.4 to 4.6 V is activated. However, Li2MnO3 has a limitation in which, past this plateau region, the increased severity of structural changes causes the electrical properties to become worse. It is known that this is due to structural changes causing a transformation from a layered structure to a spinel crystal structure such that contact between domains is loosened. Due to such properties, practical application of Li2MnO3 in batteries is difficult.
Thus, nickel-based positive electrode active materials having a discharge capacity that is at least 20% higher than cobalt-based positive electrode active materials are being actively developed. LiNiO2 has the same layered structure as LiCoO2 and has an initial discharge capacity of 180 to 200 mAh/g. However, LiNiO2 is transformed from a monoclinic structure to a hexagonal structure when charged/discharged, and thus becomes structurally unstable. Consequently, there are limitations in that the capacity rapidly decreases when continuously charged/discharged, the thermal stability and cycle properties are poor, and it is synthesizing a quantitatively stoichiometric material is difficult. In order to overcome such limitations, attempts have been at achieving structural stability by adding cobalt to LiNiO2, but since the amount of cobalt added must be at least 30 mol %, there is a limitation of causing a relative decrease in capacity.
Due to such circumstances, the material which is recently receiving the most attention as a positive electrode active material to replace LiCoO2 is a lithium nickel manganese cobalt oxide containing excessive amounts of lithium, that is, Lia1(Nix2Coy2Mnz2)2-a1O2 (where a1, x2, y2, and z2 are each independently atomic ratios of oxide composition elements and satisfy the conditions 1<a1≤1.5, 0<x2≤1, 0<y2≤1, 0<z2≤1, 0<x2+2y+z2≤1). This material is less expensive than LiCoO2 and has advantages in that the material may be used for high capacity and high voltage, but has the disadvantages of poor rate capability and poor lifetime properties at high temperatures.
To overcome such limitations, a method has been proposed for preparing a lithium transition metal oxide having a metal composition concentration gradient by synthesizing a core material and coating the exterior of the core material with a material having a different composition to prepare a double layer, and then mixing the coated core material with a lithium salt and heat treating. In this method, although the core and the external layer may be synthesized to have different metal compositions, the continuous metal composition concentration gradient is insufficiently formed in the positive electrode active material that is produced, and thus there are limitations in that the output characteristic-improving effect is unsatisfactory and reproducibility is low.